Optical system



Patented Sept. 12, 1939 JTED STATES PATENT reine OPTICAL SYSTEM tion of Germany Application January 28,-1936, Serial No. 61,13

This invention relates to an arrangement for the optic projection of an incoming television picture recreated upon the iluorescent screen of a cathode ray tube.

In the optical projection of televised pictures such as are recreated upon the fluorescent screens of cathode ray tubes, the latter in prior practice have been mostly of a kind which distinguishes itself but little so far as their construction, especially the form of the iluorescent screen, is concerned from the tubes used for the direct viewing of televised pictures recreated upon a iluorescent screen, and in this connection usually standard projector objectives of the kind generally found in the market and employed in mo- A,tion-picture equipment are used. However, what 5 Claims.

*has so far been left out of consideration is the fact that in the special problem involved in the projection of television pictures the curvature of the fluorescent screen and the curvature of the picture field in the objective of the projector machine could be suitably so adapted to each other that advantages are realizable in reference to the cost of manufacture and the quality of the recreated image upon the projection screen. The problem involved in the recreation of tele- Vision pictures by optical projection differs from the problem underlying the optical projection of a motion picture or a diapositive image in that the object to be imaged, that is to say, the fluorescent screen of a cathode ray tube need not be of a prexed form, say, in the sense that a motion-picture or diapositive image must be ilat, but that, inside certain limits, it is quite feasible to curve the fluorescent screen of a cathode ray tube and to dimension the curvature of the image held of the projection machine objeetive, in such a way that practical advantages realizable in a direction as hereinbefore indicated ccmpared with the arrangements known in the prior art.

A number of exemplied embodiments -f the invention are described in what follows by reference to the annexed drawing, in which:

Fig. 1 shows one embodiment of the invention;

2 shows another embodiment of the invention; Y

Fig. 3 shows a further embodiment of the invention;

Fig. 4 is a diagram explanatory of the invention; and

Figs. 5 and 6 are modifications of the embodiments of the invention.

Referring to Fig. 1, Ill denotes the fluorescent screen of a cathode ray tube oi which, by the way,

In Germany January 30, 1935 (Cl. Z50-164) 'tive in the absence of the lens I5.

only the truncated-cone shaped part I il of the glass bulb I I and the last deflection plate pair I2 is partly shown. I3 is a projection objective,

. and I!! is the projection surface upon which the fluorescent screen I is imaged. Using an ordinary projection objective, i. e. an objective which images the plane AB in the plane of the projection screen I li, it will be seen that the fluorescent screen I Il would be sharply imaged approximately upon the curved surface CD seeing that the fluorescent screen I0 itself is not in a plane. It thus follows that of a fluorescent screen which presents a convexity in the direction of the objective it will never be possible to produce a sharp image upon the projection screen I4. Hence, according to this invention, in combination with a fluorescent screen presenting a convexity towards the objective as shown in Fig. 1, an objective is to be used whose image leld curvature is over-compensated (corrected). In other words, such an objective would image the plane AB upon the curved surface EF and thus in turn produce from the curved object I0 a planar image upon the projection screen I4.

In the embodiment shown in Fig. 2 the fluorescent screen Ill presents a concavity towards the objective, and a projector objective is to be used which is not corrected at all, or only slightly, so far as curvature of the image ileld is concerned. By such anobjective which offers the advantage of considerably lower cost of manufacture than the standard projector objective, the curved fluorescent screen I0 will be imaged exactly in the plane of the screen I4.

In the case of the embodiment shown in Fig. 3, the fluorescent screen of the cathode ray tube is likewise curved by that directly in front of thev fluorescent screen a lens I is mounted k. wltiich presents its planar face to to the iluorescent screen and its concave face to the projector as well as in the embodiment shown in Fig. 3 one 45 could speak of a curvature of the fluorescent screen. To explain the operation of lens I5, there is indicated in Fig. 4 the part oi the fluorescent screen Il) (shown boxed by dotted lines),

lens I5, and the ray path, drawn to an exaggerated scale. A point P of the uorescent screen would send out light-rays LI and L2 to the objec- However, by mounting the lens I5 anteriorly of the screen I6 the light-rays are retracted on passing from 55 glass into air in suclra Way that outside the glass they present a larger angle in reference to the perpendicular of incidence than inside the glass. The light-rays issuing from point P, as will thus :e seen, after leaving lens l5 travel along paths as indicated by the rays L3, L4 enclosing a wider angle between each other than the light-rays Ll, L2. I

Extending rays L3, L4, into the lens l5, the point of intersection is indicated by point Pl, i. e., the point which appears to be the point P in the fluorescent screen when looking from the objective. However, the said point Pl in reference to the center of the fluorescent screen, which, with the mounting of lens l5 would not appear shifted when viewed from the objective, is placed upon a surface presenting a concavity in the direction of the objective, so that for the reasons pointed out in connection with Fig. 2 it is possibleto use in an arrangement as shown in Fig. 3 a cheap projection objective Without any correction or compensation of curvature of image eld. As shown in Fig. 5 instead of a plano-concave lens similar to the lens l5 shown in Fig. 3, it would be feasible also to employ a biconcave lens' l5- which is also mounted at close proximity to the fluorescent screen which in this instance should present a convexity to the objective. Fig. 4 holds good also for this form of lens; and the advantage of greater cheapness of the projector objective is secured in a similar Way.

The lenses l5 and IG as shown in Figs. 3 and 5 could, if preferred, be used to form the end of the cathode ray tube so that the fluorescent sub stance Would be applied directly upon the inner surface of the lens. In this case it is of great advantage to cement these lenses together with the glass Wall ll of the cathode ray tube by the use of vitreous enamel. In the part of the tube being of truncated-cone shape there is provided for this end in view a flange i7 as indicated in Fig. 6 in connection with the biconcave lens. In such a combination of lens i5 or I G and the coned part I I of the cathode ray tube, there is no risk of any change in the curvature of the lenses being occasioned inasmuch as vitreous enamel fuses at a lower temperature than glass and since it insures a high-vacuum-tight joint.

What We claim is:

l.. In a projection system, a fluorescent screen having fluorescent material mounted on a transparent refracting support, said support having a positive radius of curvature, a bi-convex lens to produce a flat image eld of the fluorescent screen, and a plane screen for receiving the produced image field.

2. In a projection system, a planar fluorescent screen, a plano-concave lens, means to support the lens with its plane surface upon the flu crescent screen, a plane screen in register with the lens, and a bi-convex lens interposed between said lens and screen.

3. In a projection system, a curved fluorescent screen, abi-concave lens, means to support the lens adjacent to and in contact with the fluorescent screen, a bi-convex lens for producing a fiat image eld of the fluorescent screen, and a plane surface for receiving the produced image field of the fluorescent screen.

4. A cathode ray tube for a projection system comprising a vitreous envelope having a surface of revolution and a flange end portion, a biconcave lens, a layer of fluorescent material on one of the cancave surfaces of the lens, and means to fasten the lens to the flange end portion of the tube with the uorescent material immediately adjacent the tube.

5. A cathode ray tube for a projection system comprising a vitreous envelope having a surface of revolution and a flange end portion, a concave lens, a layer of fluorescent material on one face of said lens, and means tc fasten the lens to the ilange end portion of the envelope with the coated surface immediately adjacent the envelope.

HANS-DIETRICH SCHMIDT-OTT.

EMIL MECHAU. 

